Screen doesn't redraw on :hover, but selection works properly

Fixed calculator example, implemented proper business logic

Implement background-size:contain and background-size:cover

…peat

This was implemented by using LayoutPrimitiveInfo's clip_rect functionality.
If `no-repeat` is specified in CSS, the rect is clipped to the size of the image on both axis.
If `repeat-x` is specified, the rect's height is clipped to the image height.
If `repeat-y` is specified, the rect's width is clipped to the image width.`

Implement background-repeat: no-repeat | repeat | repeat-x | repeat-y

Due to the instability of crates.io, this gives way better
stability and versioning guarantees with the tradeoff that
now, users might compile crates twice. However, that is
generally better than code breaking every few days, so
it's the lesser evil.

Fixes #86 and #80.

Previously, ":nth-child(1)" did not parse correctly
because the tokenizer was broken to not include the "1"
in the actual parser.

This update also adds support (in the tokenizer, not the parser)
for ::selectors, @ rules in CSS and free-standing string
literals (necessary for @ keyframes and @ media).

Necessary for handling focusing callbacks on text fields and
other, menu-related things.

- Refactored the window to use unique IDs instead of being
  dependent on the order in the Vec<Window>
- Breaking: WindowEvent: Renamed "window" field to "window_id"
- Breaking: UpdateScreen is no longer an enum, but a constant:
    - `UpdateScreen::Redraw` -> Some(()) (typedef'd to "Redraw")
    - `UpdateScreen::DontRedraw` -> None (typedef'd to "DontRedraw")

This makes it easier to clone data out of the data
model inside of a callback. Also, .modify() now
returns Option<()> in order to signify that it can fail
(and integrate with the new UpdateScreen API).

Previously things like linear-gradient(rgba()) wouldn't parse
correctly because the parser broke at the whitespaces, regardless
of the opening and closing parenthesis.

The code for implementing this is a bit messy, but it does the job.

UiState now stores all non-tag-generating callbacks in seperate
BTreeMaps, so that the event filtering can react to window,
desktop and "not" events properly.

…r events

This is a major step in order to implement focusing on text fields,
now divs can register EventFilter::Focus events that will only be
fired when the item is focused.

These changes are superficial, but serve better mnemonics -
it often happened that the "WindowInfo" and "WindowEvent"
(which isn't even a proper "event", therefore a bad name)
could be confused very easily.

Now it's easier to memorize:
- layout() function: LayoutInfo as the second argument
- callback function: CallbackInfo as the second argument

This is necessary so that a user can call functions on the
CallbackInfo, for example, in order to scroll to a certain
location or to set the focus of the app from a callback.

Correctly calculate clip_rect's origin for background-repeat

Previously, a user had to hover the cursor over the text field in
order to enter text. This is now fixed, text fields can be focused
and unfocused by clicking on them.

Users can now set the focus of the application on the next frame
using either a CSS Path (similar to JS) or a NodeId, as well as
un-focus certain

What is currently not done is that FocusLeft / FocusReceived events
are fired (in the case that the focus is set programmatically
instead of the application user doing that). This is an area that
will need to be improved in the future (likely to be implemented
when tab-navigation will be implemented).

The "actual behaviour" is usually the opposit of the "expected
behaviour", so it's fairly useless to ask for it, because most
people just put "the opposite should happen" in there.

But since both are already done in the description, it's useless
to have both.

Fix bug in background:no-repeat logic

- any_parent_has_id() / any_parent_has_class(): Check whether there
  is a parent with a certain CSS class or ID
- target_has_class() / target_has_id(): Check whether the target itself
  has a certain ID or class
- get_node() / get_node_content(): Access the DOM information of a
  certain NodeId
- get_node_hierarchy(): Returns the node hierarchy for creating custom
  iterators or custom processing / querying functions

svg.rs can serialize the BezierControlPoint, without this change
it wouldn't be able to build. serde is fairly stable and doesn't
change the API, so the chance of serde breaking the build is fairly
low - for interoperability reasons, I'd allow it to be
not version-locked.

Azul wouldn't call the default events if the given event didn't also
touch any normal events due to a copy-paste error.

Resize the GlWindow programatically to fix resizing bugs on Wayland (fixes #32).

…SvgStyle

Previously, this would properly clear the focus, but not indicate
that visually, i.e. the old :focus element was still active, even
after .clear_focus() was called.

Previously, SvgText::get_bbox() would just return (0, 0) for the
origin, now this is fixed to use the text.transform.translation
value instead.

- DisplayListParameters now carries the WindowSize, this is
  also a preparation for implementing vh, vw, vmin, vmax
- on resize events the DPI is queried from the system
  (by parsing gsettings CLI). This also makes the DPI
  respond to DPI resizing under Ubuntu.

Fixes #61.

Instead of creating the API manually or using Deref, it's
easier to create the API via a macro, so that refactoring
takes less time.

Also refactored CssImageId and ImageId to be separate from
each other - this breaks the current image API, but is a
preparation for ImageSource / FontSource.

Removed add_custom_task in favor of add_task - the Task<T>
already takes care of checking the T: Send + 'static, so
we don't have to create two separate functions for the same
thing.

This fixes the problem of override_native sometimes not being
able to override things, because the specificy sort mixes the
native and custom stylesheet in an undefined order.

.trim() and other functions got marked with #[must_use], which
breaks the #[deny(unused_must_use)] on nightly